Epitaxial Core/Shell Nanocrystals of (Europium-Doped) Zirconia and Hafnia

A careful design of the nanocrystal architecture can strongly enhance the nanocrystal function. So far, this strategy has faced a synthetic bottleneck in the case of refractory oxides. Here we demonstrate the epitaxial growth of hafnia shells onto zirconia cores and pure zirconia shells onto europium-doped zirconia cores. The core/shell structures are fully crystalline. Upon shelling, the optical properties of the europium dopant are dramatically improved (featuring a more uniform coordination and a longer photoluminescence lifetime), indicating the suppression of nonradiative pathways. These results launch the stable zirconium and hafnium oxide hosts as alternatives for the established NaYF4 systems.

Z irconium and hafnium oxide nanocrystals (NCs) are appealing (host) materials due to their high thermal and chemical stability and large band gap. 1 Colloidal ZrO 2 NCs are components for (in)organic composites, 2−4 while HfO 2 NCs find applications in memory devices. 5,6−13 Both ZrO 2 and HfO 2 NCs serve as hosts for optically active lanthanide ions, e.g., europium. 1,14−23 In the fluoride system, the syntheses are well-developed, allowing for the precise positioning of dopants inside the nanocrystal and the growth of undoped shells on the doped core.The latter results in core/ shell architectures, which were pioneered in the field of semiconductor nanocrystals (quantum dots) to prevent the excited electrons and holes from interacting with surface traps. 24,25Likewise, shelling protects the lanthanides from surface effects and thus increases the quantum efficiency of both up-and downconversion processes. 26Additionally, multilayered structures offer controlled energy cascades in the case of lanthanide-doped fluorides. 27Higher quantum efficiencies coupled with long lifetimes enabled their use in, e.g., time-gated fluorescence imaging. 15,28The oxide hosts have found less widespread use because of the synthetic challenge of producing colloidally stable oxide nanocrystals with a complex (e.g., core/shell) architecture. 29However, the oxide hosts are more chemically stable, while the fluorides dissolve in highly dilute aqueous media. 30urfactant-assisted, nonaqueous synthesis methods have allowed the synthesis of ZrO 2 and HfO 2 NCs with control over their size. 1,31−37 Even more, solid solutions of various compositions (Zr x Hf 1−x O 2 ) were obtained as colloidal nano-crystals. 34,38,39Hf 0.5 Zr 0.5 O 2 is an interesting ferroelectric material, making it promising for nonvolatile memory devices. 40,41ere we leverage the control of nonaqueous synthesis and report metal oxide core/shell nanocrystals.We epitaxially grow HfO 2 (or ZrO 2 ) onto ZrO 2 and ZrO 2 onto ZrO 2 :Eu 3+ .The beneficial effect of the shell on the optical properties of the europium dopant is established.Having access to these novel heterostructures, which are unavailable in bulk, opens up possibilities for their application in different areas, from microelectronics to scintillators.
Zr and Hf are chemically very similar, and both oxide nanocrystals can be synthesized at 340 °C from the metal chloride and isopropoxide in tri-n-octylphosphine oxide (TOPO). 1 Under these conditions, HfO 2 forms nanorods with the monoclinic crystal structure, while ZrO 2 forms spherical nanocrystals with the tetragonal crystal structure.We first focus on the ZrO 2 /HfO 2 core/shell structure, since one can conveniently distinguish growth of HfO 2 onto the spherical ZrO 2 cores from separately nucleating HfO 2 nanorods.ZrO 2 and HfO 2 form similar crystal structures (monoclinic, tetragonal, and cubic) with negligible lattice mismatch, thus allowing for epitaxial growth.We followed a two-step approach toward the core/shell structure.In the first step, we synthesized and isolated ZrO 2 cores from ZrCl 4 and Zr(O i Pr) 4 • i PrOH in TOPO (Figure 1A). 42The cores are spherical with an average diameter of 3.8 nm according to bright-field transmission electron microscopy (BF TEM) (Figure 1B).The cores have the tetragonal (P4 2 /nmc) crystal structure (Figures S1 and S2), and their surface is covered with a mixture of protonated TOPO, dioctyl phosphinate, and dioctyl pyrophosphonate (Figure 1D). 42In a second step, the purified cores were added to a new reaction mixture of HfCl 4 (THF) 2 and Hf(O i Pr) 4 • i PrOH in TOPO (see Figure 1A).After 2 h at 340 °C, the core/shell structures were isolated and purified.The nanocrystal surface is covered by the same ligands as mentioned before (see Figure 1D for the 31 P NMR spectrum).Atomic-resolution high-angle annular darkfield (HAADF) scanning TEM (STEM) analysis shows that the resulting nanocrystals are no longer spherical but more clearly faceted, exposing {011̅ } and {101̅ } surfaces (Figures S3  and S4).To be able to compare the sizes of core and core/shell particles, we analyze the BF TEM image (Figure 1C) by measuring the projected area of the faceted nanocrystals and calculating an apparent "diameter", which is the diameter for a circle with equal area.The core/shell structures have an average "diameter" of 4.9 nm, clearly indicating that the cores (d = 3.8 nm) have grown to larger sizes.Crystal growth is further confirmed by the narrower reflections in X-ray diffraction (XRD) (Figure S1) and extended features in the pair distribution function (PDF) (Figure S2).
The core/shell interface was further analyzed by atomicresolution HAADF STEM (see Figures 2, S3, and S4).From the contrast in Figure 2A, we can recognize the presence of a core/shell structure, as the shell appears brighter with respect to the core due to the higher atomic number of Hf compared to Zr.The power spectrum (FFT) analysis in the reciprocal space shows that the ZrO 2 core has the P4 2 /nmc tetragonal structure and that the HfO 2 shell has a similar structure.Tetragonal HfO 2 has the following lattice parameters at room temperature: a = b = 3.65 Å, c = 5.33 Å.These are slightly higher than those of tetragonal ZrO 2 : a = b = 3.59 Å, c = 5.18 Å. 43,44 This is in good agreement with the observed crystal arrangement in our core/shell nanoparticles.The (101) and (101̅ ) reflections are "elongated" or doubled, indicating the presence of HfO 2 that grows epitaxially and partially relaxed onto the indicated surfaces of the ZrO 2 core.The frequencyfiltered map clearly displays the core in green and the shell in red.
While pure HfO 2 nanocrystals would crystallize in the monoclinic P2 1 /c structure, epitaxial growth thus stabilizes hafnia in its tetragonal structure.Final confirmation of the core/shell structure is provided by energy-dispersive X-ray spectroscopy (EDX) compositional mapping, which shows regions with zirconium in the center and hafnium in the shell (Figure 2B).When shelling the zirconia cores with zirconia shells under identical conditions, we cannot infer the success from compositional mapping.However, the final core/shell particles had an average "diameter" of 5.5 nm, indicating successful growth (Figure S5).
Second, we turn to the optically active ZrO 2 :Eu/ZrO 2 core/ shell system.We synthesized europium-doped zirconia (10.0%nominal doping) from europium acetate, zirconium propoxide, and benzyl alcohol (Figure 3A). 15After synthesis, we functionalized the nanocrystal surface with dodecanoic acid ligands (Figure S6).The ZrO 2 :Eu cores have an average diameter of 3.5 nm (Figure 3B), possess the tetragonal crystal structure (Figures S1 and S2), and have an actual doping concentration of 9.0% (determined by ICP-OES).−17 The cores were subjected to a shelling procedure using ZrCl 4 (THF) 2 and Zr(O i Pr) 4 • i PrOH (Figure 3A).After 2 h at 340 °C, colloidally stable nanocrystals were isolated and purified (Figure S7 for 31 P NMR and Figure S8 for DLS).BF  TEM imaging shows an increase in the average "diameter" to 5.1 nm (Figure 3C), and again, sharper reflections in XRD and extended signals in the PDFs are observed, confirming growth of the tetragonal crystal (Figures S1 and S2).HAADF STEM and powder spectrum analysis further confirm that both ZrO 2 :Eu cores and ZrO 2 :Eu/ZrO 2 core/shells have the tetragonal structure (Figures S9 and S10).The europium content decreases in the core/shells to 1.8% metal content.This is close to the expected 2.1% considering the stoichiometry of the reaction.
The optical properties of the europium dopant change substantially when the cores are shelled (see Figures 4A, S11,  and S12).The excitation and emission spectra of Eu in the ZrO 2 :Eu cores is consistent with previous reports of Eu doped into colloidal ZrO 2 nanocrystals or ZrO 2 powders. 15,17,45,46pon shelling, certain emission channels disappear (e.g., the emission band at 612 nm, belonging to a 5 D 0 → 7 F 2 transition), and the remaining emission peaks are more narrow (Figure 4A).This indicates a change in the coordination around the europium ion.This is further demonstrated by the shape of the 5 D 0 → 7 F 0 transition around 580 nm, measured at 10 K (Figure S13).This J = 0 → J′= 0 transition is not split by the crystal field, and thus, spectrally different contributions for this transition can be assigned to different coordinating environments.For the core/shell, the emission band corresponding to this transition is narrower and more symmetric, indicating a highly uniform environment for europium.
The shell also has a significant impact on the lifetime of the excited state (Figure 4B).For the ZrO 2 :Eu cores, the luminescence decay depends on the monitored emission wavelength but in all cases is biexponential.This variation in lifetime points again to different europium coordination environments.We fit the decay profiles using the function At 606 nm, we determine a slow component of τ 1 = 2.7 ± 0.1 ms and a fast component of τ 2 = 1.2 ± 0.1 ms.The slow component contributes 86% to the total emission, calculated via For the core/shells, the decay is monoexponential and independent of the monitored wavelength (Figure 4B).We determined a lifetime of 5.3 ± 0.1 ms.This is similar to the longest values reported for Eu 3+ in ZrO 2 (nano)powders, 47 indicating that the contribution of nonradiative decay, e.g.via interaction with surface or other defects, is limited.In the literature, long lifetimes of 4−5 ms appear only for low doping percentages (1%).Higher doping percentages are typically associated with shorter lifetimes. 47Here we report a core doped with 9% Eu that features a lifetime of 5.3 ms after shelling and thus passivation of the surface defects.Some authors attribute the faster decay to a monoclinic crystal phase and the slower decay to the tetragonal phase. 48Given that we find no evidence of a monoclinic phase, we assign the fast decay (present only in the ZrO 2 :Eu cores) to Eu ions that are near the nanocrystal surface.Based on the changes in the emission spectrum and the increase in lifetime, we conclude that the shelling improves the incorporation of the Eu ions in the oxide crystal.−55 The coexistence of broadband and europium emission in ZrO 2 (with 0.4% Eu doping) was recently exploited for ratiometric fluorescence thermometry in the range from 130 to 230 K. 56 We did not observe this strong broadband emission for the ZrO 2 :Eu cores with high Eu doping (9%) (see Figure S14), implying that the presence of Eu ions suppresses the defect emission.This suggests that for  the core/shell particles, the Eu ions remain located in the core during the shelling, since the presence of Eu ions in the shell would suppress the ZrO 2 defect emission (either by preventing the defect from forming or by short-range energy transfer to Eu). 57−59 Therefore, the overall emission spectrum of the core/shells contains emission from Eu 3+ in the core and defect emission from the shells.This also explains the drastic increase in the Eu 3+ lifetime of the core/shell particles, since the Eu 3+ ions are now in a perfect environment, shielded from the surface.
In summary, we have reported the use of crystalline zirconium or hafnium oxide to shell nanocrystals.This method was applied to improve the local environment of europium dopants in zirconia nanocrystals and to suppress nonradiative de-excitation pathways.Our results are an invitation to explore intricate nanocrystal architectures with oxide host materials.

Figure 2 .
Figure 2. (A) HAADF STEM image of a core/shell ZrO 2 /HfO 2 nanocrystal, the corresponding power spectrum (FFT) analysis in the reciprocal space along the [010] zone axis, and the frequency-filtered map HAADF STEM image.(B) HAADF STEM image of several ZrO 2 /HfO 2 nanocrystals and the corresponding EDX compositional maps featuring hafnium and zirconium.

Figure 4 .
Figure 4. (A) Photoluminescence emission spectra and (B) lifetime decays at various emission wavelengths of ZrO 2 :Eu cores and ZrO 2 :Eu/ZrO 2 core/shells, excited at 238 nm.The measurements were carried out at room temperature in cyclohexane with an absorbance of 0.1 at 238 nm.